In one aspect of utilizing solar energy, a solar cell is used to convert solar energy into electric energy. The power of the solar cell differs according to the strength of a solar light incident upon the solar cell and to an ambient temperature, as indicated in FIGS. 1 and 2. These variations in the power bring about a change in an output impedance of the solar cell. If a fixed load is driven by the solar cell, impedance mismatching occurs and the efficiency of the power transfer to the load is lowered. Hence, efforts have been made to maximize the power transfer efficiency.
In U.S. Pat. No. 4,873,480, incorporated herein by reference, a cell array and an independent cell are installed in a solar cell panel, as shown in FIG. 3. A reference voltage is generated through the independent cell, and a pulse width modulation signal is generated by comparing the reference voltage with an output voltage of the cell array. The power supplied to a load is switched in response to pulse width modulation signal. Therefore, the output voltage of the solar cell is maintained at a constant voltage level irrespective of the strength of the solar light or the ambient temperature.
However, a technique such as that disclosed in the above U.S. Pat. No. 4,873,480 causes slight variations in the output voltage in order to obtain the maximum power point for the level of solar light, as indicated in FIG. 1. Although the efficiency is better than when a constant reference voltage is used, it is difficult to transfer the maximum power at every condition, and an additional independent cell should be provided.
To compensate for variations in the output voltage of a solar cell due to ambient temperature fluctuations, U.S. Pat. No. 4,580,090, incorporated herein by reference, utilizes a thermistor for detecting the output voltage of the solar cell and compensating the detected voltage for temperature, as shown in FIG. 4. However, the output voltage of the solar cell is detected by dividing the voltage through resistors, and the temperature is compensated by using the thermistor connected in series with these resistors. Therefore, if the solar light is very weak, the overall efficiency is lowered.
In U.S. Pat. No. 4,916,382, incorporated herein by reference, the output voltage and current of the solar cell are converted to digital data through an analog-to-digital converter, as shown in FIG. 5. The digital data obtained is processed by a controller to calculate and store the maximum power point. However, such a technique requires complicated circuitry due to the input power measurement interface circuit needed for the controller, which raises the cost.